1. Field of the Invention
The present invention relates generally to detecting RF field strength, and, in particular, to detecting RF field strength in a passive RFID system.
2. Description of the Related Art
In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art that should be familiar to those skilled in the art of radio frequency (“RF”) communication systems. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic_0 and logic_1. Of course, as is well know, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
In accordance with our prior invention previously disclosed in the Related References, the amplitude modulated (“AM”) signal broadcast by the reader in an RFID system will be electromagnetically coupled to a conventional antenna, and a portion of the current induced in a tank circuit is extracted by a regulator to provide operating power for all other circuits. Once sufficient stable power is available, the regulator will produce, e.g., a power-on-reset signal to initiate system operation. Thereafter, the method disclosed in the Related References, and the associated apparatus, dynamically varies the capacitance of a variable capacitor component of the tank circuit so as to dynamically shift the fR of the tank circuit to better match the fC of the received RF signal, thus obtaining maximum power transfer in the system.
In general, the invention disclosed in the Related References focused primarily on quantizing the voltage developed by the tank circuit as the primary means of matching the fR of the tank circuit to the transmission frequency, fC, of the received signal. However, this voltage quantization is, at best, indirectly related to received signal field strength. We submit that what is needed now is an effective and efficient method and apparatus for quantizing the received field strength as a function of induced current. It is further desirable to develop this field quantization in a form and manner that is suitable for selectively varying the input impedance of the receiver circuit to maximize received power, especially during normal system operation. Additionally, in light of the power sensitive nature of RFID systems, it is desirable to vary the input impedance with a minimum power loss.
BRIEF SUMMARY OF THE INVENTION
In accordance with the preferred embodiment of our invention, we provide an RF communication system in which an external transmitter electromagnetically couples a signal of predetermined frequency, fC, to a receiver separated by an air gap from the transmitter. The system uses a method for dynamically maximizing a received power transfer by adjusting a tuning circuit of the receiver. The tuning circuit comprises an antenna and a capacitor that can be varied in a selected one of first and second directions. The method comprises the steps of (1) choosing a selected one of said first and second directions; (2) capturing a first current response of the tuning circuit to the signal transmitted by the transmitter; (3) varying the capacitor in the selected first or second direction; (4) comparing the captured first response to a second current response of the tuning circuit to the signal currently transmitted by the transmitter; (5) if the comparison made in step 4 indicates that the second current response is weaker than the captured first response, selecting the other of the selected directions; and (6) returning to step 2.
In accordance with another embodiment of our invention, we provide a power detector for use in an RF receiver. The power detector comprises a power reference generator and a power quantizer. The power reference generator is adapted to develop a power reference current as a function of a power transferred via a received RF signal. The power quantizer is responsive to the power reference current and is adapted to develop a digital field power value indicative of the power reference current.
In accordance with another embodiment of our invention, we provide a field strength quantizing detector for use in an RF system. The detector comprises a regulator, a current source circuit, a reference circuit, and a control circuit. The regulator circuit is adapted to conduct a first current proportional to the field strength of a received electromagnetically coupled RF signal. The current source circuit is adapted to develop a field strength reference current in response to a digital input. The reference circuit is coupled to the regulator circuit and to the current source circuit and is adapted to develop a mirrored current as a function of the first current and the field strength reference current. The control circuit adapted to capture the mirrored current, compare the captured mirrored current against a predetermined threshold value, increase the field strength reference current in response to an indication that the field strength current is insufficient, and cease operation in response to an indication that the field strength reference current is sufficient.
In accordance with another embodiment of our invention, we provide a method of quantizing field strength in an RF system comprising a regulator, a current source, a reference circuit, and a control circuit. The method comprises the steps of (1) conducting a first current proportional to the field strength of a received electromagnetically coupled RF signal, (2) developing a field strength reference current in response to a digital input, (3) developing a mirrored current as a function of the first current and the field strength reference current, (4) capturing the mirror current, (5) comparing the captured mirrored current against a predetermined threshold value, (6) changing the field strength reference current in response to an indication that the field strength current is insufficient, and (7) ceasing operation in response to an indication that the field strength reference current is sufficient.
In accordance with another embodiment of our invention, we provide a field strength quantizing detector for use in an RF system. The detector comprises a regulator circuit, a current source, a reference circuit, and a control circuit. The regulator circuit is adapted to conduct a first current proportional to the field strength of a received electromagnetically coupled RF signal. The current source circuit is adapted to develop a field strength reference current in response to a digital input and to develop a reference voltage on a sensing node in response to said field strength current. The reference circuit is coupled to the regulator and to the current source and is adapted to develop a mirrored current as a function of the first current and the field strength reference current and to further develop the reference voltage on the sensing node in response to the mirrored current. The control circuit is adapted to capture the reference voltage, compare the captured reference voltage against a predetermined threshold value, change the field strength reference current and further develop the reference voltage in response to an indication that the field strength current is insufficient, and cease operation of the control circuit in response to an indication that the field strength reference current and the reference voltage are sufficient.
In accordance with another embodiment of our invention, we provide a method of quantizing field string in an RF system comprising a regulator, a current source, a reference circuit, and a control circuit. The method comprises the steps of (1) conducting a first current proportional to the field strength of a received electromagnetically coupled RF signal; (2) developing a field strength reference current in response to a digital input and developing a reference voltage on a sensing node in response to the field strength current; (3) developing a mirrored current as a function of the first current and the field strength reference current and further developing the reference voltage on the sensing node in response to the mirrored current; (4) capturing the reference voltage; (5) comparing the captured reference voltage against a predetermined threshold value; (6) changing the field strength reference current and further developing the reference voltage in response to an indication that the field strength current is insufficient; and (7) ceasing operation in response to an indication that the field strength reference current and the reference voltage are sufficient.
In accordance with another embodiment of our invention, we provide a maximum current detection circuit in an RF system. The maximum current detection circuit comprises a current shunt circuit, a current mirror circuit, an analog-to-digital converter (“ADC”), a digitally controlled current source (“DCCS”), and a digital logic block. The current shunt circuit is adapted to conduct a first current proportional to a received signal. The current mirror circuit is coupled to the current shunt circuit and is adapted to conduct a second current that is proportional to the first current and to produce a reference voltage that is proportional to said second current. The an a ADC is coupled to the current mirror circuit and is adapted to generate a first digital value if the reference voltage is below a first voltage value, to generate a second digital value if the reference voltage is above a first voltage value and output an ADC value that is a selected one of the first digital value and second digital value. The DCCS is coupled to the current mirror and the ADC and adapted to modify the reference voltage in response to receiving a DCCS code. The digital logic block is coupled to the ADC and the DCCS and is adapted to sweep the DCCS code from a first value to a second value, to save a first copy of the DCCS code corresponding to the first instance of ADC value indicating the reference voltage is above the first voltage value.
In accordance with another embodiment of our invention, we provide a tuning circuit for dynamically varying the impendence of a tank circuit. The tank circuit comprises an inductor and a capacitor, the capacitance of which can be varied in a selected one of first and second directions. The tuning circuit comprises a voltage regulator, a current mirror, an analog-to-digital converter (“ADC”), a digitally controlled current source (“DCCS”) and a digital logic block. The voltage regulator is coupled to the tank circuit and is adapted to maintain a constant voltage. The voltage regulator further comprises a current shunt circuit adapted to conduct a first current proportional to a received signal. The current mirror circuit is coupled to the current shunt circuit and is adapted to conduct a second current that is proportional to the first current and to produce a reference voltage that is proportional to the second current, The ADC is coupled to the current mirror circuit and is adapted to generate a first digital value if the reference voltage is below a first voltage value, to generate a second digital value if the reference voltage is above the first voltage value and output an ADC value that is a selected one of the first digital value and second digital value. The DCCS is coupled to the current mirror and the ADC and is adapted to modify the reference voltage in response to receiving a DCCS code. The digital logic block is coupled to the voltage regulator and the tank circuit and adapted to save a first copy of the DAC code corresponding to the first instance of ADC value indicating the reference voltage is above the first voltage value, to sweep the DCCS code from a first selected value to a second selected value, to determine a polarity of the change in a previous value of the DCCS code and current value of the DCCS code, to select one of the first and second directions in response the polarity and to selectively vary the capacitance of the capacitor in the selected direction.
In the drawings, similar elements will be similarly numbered whenever possible. However, this practice is simply for convenience of reference and to avoid unnecessary proliferation of numbers, and is not intended to imply or suggest that our invention requires identity in either function or structure in the several embodiments.